APOBEC3 (A3) proteins are host cellular proteins that confer intrinsic immunity to viral infections. Human have seven A3 proteins, including human A3 (hA3G) and hA3F. hA3G restricts infection of cells in culture by Vif-deficient human immunodeficiency virus 1 (HIV-1), primate foamy virus (PFV), hepatitis B virus (HBV), murine leukemia virus (MuLV), and mouse mammary tumor virus (MMTV). In contrast to humans, the mouse genome encodes a single A3 gene (mA3) which restrict infection by HIV-1, MuLV, and MMTV. Recently, we provided the first in vivo evidence that A3 is an antiviral restriction factor. By comparing MMTV infection in mice deficient in mA3 and their mA3-sufficient counterparts, we showed that mA3 restricted infection by this virus in vivo. More recently, we showed that mA3 restricts MMTV infection in dendritic cells (DCs), the initial targets of infection by many retroviruses. The studies proposed here will take advantage of the MMTV-mA3 model system to explore the relationship between mA3 and MMTV-induced pro-inflammatory cytokine induction. Recently, it has been shown that a number of cytokines induce expression of hA3G and hA3F in cultured cells. It is well-established that MMTV induces cytokine production at two steps in the infection pathway: first, when the virus envelope protein interacts with the innate immune receptor Toll-like receptor 4 (TLR4) and second, when virus-infected antigen presenting cells such as DCs present the viral superantigen (Sag) to T cells. Thus, it is likely that MMTV infection also triggers cellular expression of mA3. Using ex vivo cultures of DCs from mA3-/- and mA3+/+ mice, we will investigate if cytokine treatment induces mA3 expression and the effect of such induction on MMTV infection. We will also carry out animal studies and examine cytokine induction, mA3 expression and virus restriction in vivo. To determine at which step in the infection pathway mA3 is induced, we will use mice lacking mA3, MyD88, TRIP and TLR4 genes and mice unable to respond to the viral Sag protein. Because of its antiviral properties, higher mA3 expression during the early stages of infection may serve to allow the host's immune system time to limit virus infection. RELEVANCE: The proposed studies will use MMTV-mA3 model system to explore the regulatory mechanism(s) of mA3 and its interaction with the broader immune system. Indeed, it has been suggested that increasing hA3G expression through the use of agents that induce cytokine production could be used as an anti-HIV-1 therapy. Thus our studies will increase understanding of the cross-talk between A3 and the immune system and will set the stage for the use of this protein as a means of developing new anti-retroviral therapies